1. Field of the Invention
The invention relates to a process for making semi-finished products in the form of thin metal bars in accordance with the introductory part of Patent Claim 1 and to a device for implementing the process and a device bars in which, an uncooled, cleaned metal profile having a low heat content is run continuously from the bottom to the top through a melt pool of material of the same composition.
2. Description of the Prior Art
For this purpose, there is a slot-type opening in the bottom of the melt container that is equipped with a sealing device in for preventing the melt from flowing out. The temperature of the melt lies in the vicinity of the liquidus temperature T.sub.liq. The steel strip moves through the melt at a constant speed and passes out of the melt in the upward direction. Because of the low heat content of the steel strip (strip temperature is approximately equal to room temperature), an adherent layer of crystallized and still molten melt develops on its surface. The thickness of this layer may be several times the thickness of the original blank. The thickness of the layer depends, on retention time in the melt (speed of blank), the melt temperature (temperature difference relative to the solidus temperature T.sub.sol), the melt heat and the specific heat of the material used, and the thickness of the blank. The operation must be conducted in such a manner as to avoid remelting the already adherent crystalline like. Under these conditions, a temperature gradient is induced across the thickness of the strip. As the strip moves through the melt pool, the temperature is lowest in the interior of the blank and rises toward the edge. A qualitatively similar temperature curve is also present in the adherent layer. The temperature in the outermost region of the layer is the liquidus temperature T.sub.liq (mushy zone). The proportion of the molten phases in the layer increases in a direction toward the melt. After leaving the melt pool, the blank and the adherent layer cool, whereby the temperature gradient that has existed until now is reversed. The adherent layer then solidifies completely.
EP 0 311 602 B1 also discloses that the semi-finished product produced as described above kept, after it leaves the melt pool and until it cools or enters a forming machine to undergo a hot or cold forming process is to be kept, in an atmosphere protection against oxidation. A portion of the total amount of finished product produced in this manner is then fed back to the start of the process as blank and run through the melt pool once again.
Until now, a crucial obstacle has hindered the practical application of this process in making steel strip material. Consumers of high-quality cold or hot strip demand; among other things, that the range of deviation in sheet metal thickness be no greater than 2% of the nominal thickness. A tight tolerance of this kind cannot be reliably maintained using the aforementioned process. Irregularities in strip thickness which exist after the strip left the melt pool and which exceed the prescribed maximum limit are practically impossible to eliminate by means of subsequent forming procedures. This is because, given the extreme flatness of the semi-finished product used in the rolling process (width-gauge ratio of at least 60), the subsequent forming (with decreasing thickness) takes place, essentially, in the longitudinal direction only; no further significant widening occurs. Existing differences in thickness; along a line at a right angle to the longitudinal direction of the strip; therefore relatively, unchanged.
EP 0 311 602 B1 also describes another embodiment of the process wherein, in a reverse fashion, the blank is introduced into the melt pool from above and then drawn through the bottom of the melt vessel. The problem of sealing the bottom is particularly serious for the because the outflow directions of the melt and the strip material are the same. As a result, not only is there no dynamic sealing effect, but there is also carry along which helps a negative "effect" that supports induce the melt to flow out of the vessel. For this reason, a special sealing device in the form of a sealing roll pair is positioned in the bottom region of the melt vessel. This sealing roll pair causes a drastic compression of the "mushy zone," and thus large portions of the molten phase are squeezed out of the already formed "spongy" crystalline like formation. Consequently, the thickness attainable in the adherent layer, compared to the first embodiment, is considerably, smaller. For economic considerations alone, such a process is unsuitable for practical applications.